1. Field of the Invention
The present invention relates to magnetic recording disks used in hard disk drives (HDDs), and more particularly, to a perpendicular magnetic recording disk with improved thermal stability of record information and high signal-to-noise (SNR) ratio.
2. Description of the Related Art
In longitudinal magnetic recording (LMR) applied to hard disk drives (HDDs), a major external data storage device of computers, the size of a data record domain in a magnetic disk has decreased with microstructure as the need for high-density data recording increases. However, this decrease in size makes the data record domains susceptible to removal by thermal energy generated by operation of the HDD which is more dominant than magnetostatic energy from the data record domain. This is referred to as the super paramagnetic effect. To overcome the super paramagnetic effect, the LMR technique has been replaced by a perpendicular magnetic recording (PMR) technique for HDD applications. The PMR technique uses a higher electrostatic energy and lower demagnetization energy compared to the LMR technique, so it is advantageous in high-density data recording. The high-density PMR technique also has enabled detection of a micro data domain in combination with advances in the manufacture of highly sensitive read heads.
In the PMR technique suitable for high-density magnetic recording, perpendicular magnetic anisotropy energy is exerted to orient the direction of magnetized domains perpendicular to the plane of a magnetic disk. Thus, head fields from a magnetic head should be induced to be perpendicular to the magnetic disk plane and thus parallel to the magnetized domains. To achieve this, a particular head capable of inducing a perpendicular magnetic field is required.
The schematic structure of a single-layer PMR medium is shown in FIG. 1. The single-layer PMR medium includes an underlayer 12 for promoting the perpendicular orientation of a perpendicular magnetic recording (PMR) layer 13 formed over the underlayer 12, the PMR layer 13 having the perpendicular magnetic anisotropy energy to keep the perpendicular orientation of the data record domain, a protective layer 14 for protecting the PMR layer 13 from external impacts, and a lubricant layer 15.
The PMR layer 13 has the perpendicular magnetic anisotropy energy with a magnetic easy axis oriented perpendicular to the plane of the PMR layer 13 due to the underlayer 12. Therefore, perpendicular data recording can be achieved by perpendicular magnetic field components from a ring-type head.
Recording density in a perpendicular magnetic recording mechanism is largely affected by the characteristics of the PMR layer and the perpendicular orientation promoting underlayer (hereinafter, simply referred to as “underlayer”).
In a conventional PMR medium, as shown in FIG. 1, an underlayer disposed below a PMR layer to promote perpendicular magnetic orientation of the PMR layer is formed of titanium (Ti), platinum (Pt), gold (Au), or palladium (Pd), which has good perpendicular magnetic orientation properties. In this case, crystal growth in the underlayer is continuous to the PMR layer in an aligned grain structure, thereby increasing the grain size of the PMR layer. Such large grains result in greater noise with low signal-to-noise ratio (SNR).